Duplex and Half Duplex transmission schemes are used in a variety of voice and data services. Duplex voice schemes are generally perceived by subscribers as providing better service quality than half duplex, whereas half duplex voice schemes provide a more efficient use of system resources than duplex schemes.
FIG. 1 is a diagram representative of typical voice traffic operating over a half duplex channel. In such a scheme the listening party is prevented from transmitting a voice signal until the talking party has ceased transmitting.
FIG. 2 is a diagram representative of typical voice traffic on a duplex channel. The parties to the call have intermittent periods where both parties are talking. These periods may be considered as contention periods. Typically the parties to the call resolve these contention periods between themselves so that a mutually understandable conversation results. It is unusual for both parties to talk at the same time for an extended period.
Duplex transmission schemes have the advantage over half duplex schemes of improved call quality for subscribers. However they have the disadvantage of inefficient use of system resources, as typically one channel will be idle at any one point during a conversation.
One example of a duplex channel is a time division duplex (TDD) arrangement where both parties have alternating access to the channel. Typically, the channel may have a capacity of 72 Kbits.sup.-1, in an ISDN system. Each party is typically sampled at 32 Kbits.sup.-1 for 2 ms periods. These sampled signals are transmitted along the channel at the higher data rate of 72 Kbits.sup.-1 using 1 ms packets of data.
However with typical voice traffic, approximately half of the time slots on the TDD channel will be idle.
WO 95/04412 to Motorola provides a method and apparatus for multiplexing fixed length message data and variably coded speech. A variable rate vocoder receives and codes the speech of an unknown amount at one of full and less than full rates to produce the coded data. U.S. Pat. No. 5,124,701 to France Telecom provides a quantisation device with a variable digital coding rate.